The present invention relates to hydrodynamic reversing transmissions, and more particularly to improvements in hydrodynamic reversing transmissions which can be utilized with advantage in earth moving vehicles, forklift trucks, locomotives for shunting or switching service in marshalling yards, or other types of heavy-duty conveyances. Still more particularly, the invention relates to improvements in hydrodynamic reversing transmissions of the type wherein each of the forward and reverse drive trains comprises at least one hydrodynamic torque converter.
Commonly owned U.S. Pat. No. 4,184,330 granted Jan. 22, 1980 to Polzer et al. discloses a hydrodynamic reversing transmission wherein the input shaft receives torque from an internal combustion engine and the output shaft transmits torque to the wheels of an earth moving vehicle or the like. The vehicles which can utilize hydrodynamic reversing transmissions, e.g., transmissions of the type disclosed in the patent to Polzer et al., will often move back and forth, for example, in a first direction to fill the bucket of a scraper and thereupon in the opposite direction to move the filled bucket to an unloading station. An advantage of a hydrodynamic reversing transmission in a vehicle of the just outlined character is that the wear upon the parts of the transmission is practically nil when the operator decides to change the direction of movement from forward to reverse or vice versa. Thus, all that is necessary is to drain the torque converter or converters which was or were operatively connected with the output shaft of the transmission and to simultaneously fill the torque converter or converters which are about to transmit torque to the output shaft. The switchover (i.e., draining of the active torque converter or converters and simultaneous filling of the previously inactive torque converter or converters) can take place while the vehicle is in motion whereby the transmission performs a desirable braking action, i.e., the torque converter or converters which are in the process of being filled oppose further movement in the direction in which the vehicle is moving during such switchover. This can (and hereinafter will) be called the counterbraking range or the range within which the turbine of a converter operates in the negative direction.
In order to achieve a high starting torque, each of the at least two torque converters of the hydrodynamic reversing transmission which embodies the present invention preferably constitutes a starting converter, i.e., the optimum efficiency of such converters is accomplished when the ratio (n.sub.T /n.sub.P) of RPM of the turbine wheel to RPM of the pump wheel is between 0.35 and 0.6. Rapid reaction of the hydrodynamic reversing transmission to signals which are initiated by the operator of the vehicle is ensured if the transmission includes or is combined with a reversing valve of the type disclosed in the aforementioned patent to Polzer et al. Moreover, the reversing transmission receives working fluid from a large-volume hydrostatic filling pump which is designed to deliver working fluid at a substantially constant rate under all operating conditions and is installed in a so-called open hydraulic circuit.
It has been found that the aforementioned reversing transmission of Polzer et al. can present problems during certain stages of operation, for example, when a torque converter is utilized for hydrodynamic braking within the counterbraking range. It is particularly difficult to achieve a satisfactory progress of braking moment within the entire range of rotational speeds of the turbine wheel, i.e., a progress which is satisfactory when the transmission is in actual use. It is already known to select various braking moments by adjusting the positions of adjustable guide vanes (or by otherwise influencing the torus- or doughnut-shaped flow of working fluid) within the range of relatively low negative turbine wheel speeds. The characteristic curves of braking moments at different positions of the guide vanes preferably form a pattern of fanwise arranged curves. However, when the negative turbine wheel speed (and more particularly the ratio n.sub.T /n.sub.P of RPM of the turbine wheel to RPM of the pump wheel) is relatively high, the characteristic curves which denote the braking torque and correspond to different positions of adjustable guide vanes extend in part downwardly and in part upwardly as the aforementioned ratio n.sub.T /n.sub.P increases, i.e., the characteristic curves intersect each other. This renders it practically impossible to select different braking moments for various speeds of the vehicle which embodies the patented reversing transmission. Furthermore, when the aforementioned ratio is relatively high, the braking moment which is furnished in response to movement of adjustable guide vanes to certain positions is much too high. This renders it necessary to undertake additional steps in order to limit the braking moment.